It is known to use forklift trucks to remove and place loads on surfaces of varying depths and heights. Such forklifts generally comprise a wheeled chassis on which is mounted an upright mast and means for carrying loads. Usually the means for carrying loads are in the form of L-shaped members such as forks or tines that are able to engage the load to be carried. For the purpose of this specification and unless otherwise noted explicitly, the terms load carrying means, forks or tines shall be used interchangeably to describe the means by which a forklift truck carries its load. It is also known that such forklift trucks can be adapted to be mounted on a carrying vehicle. These forklift trucks are conventionally known as ‘truck mounted’ forklifts or ‘piggy-back’ forklifts.
Conventional forklifts are rated for loads at a specific maximum weight when at a specified forward centre of gravity. The forklift and load are regarded as a unit that has a continually varying centre of gravity with every movement of the load. Accordingly, all forklift trucks have to be designed to provide enough counterbalance to counteract the tipping moment caused by lifting the specified rated load capacity for stacking. More importantly the forklift truck must also have enough counter-balancing weight for travelling mode where the dynamic forces experienced require greatly increased stability.
Conventional counterbalance forklifts carry extra counterbalance weight on the rear of the truck to ensure safe operation while stacking or travelling. However, truck mounted forklifts are generally of straddle frame construction which enables the load to be carried substantially between the front wheels during travelling mode. This greatly improves stability without the requirement for additional counterweight. However, straddle frame construction generally requires a reach system to enable the forks to engage the load especially on a trailer bed or raised platform.
Generally, reach systems comprise, for example, moving mast systems, telescopic forks or pantograph linkage arrangements. When the forks are in an extended position, the load capacity that can be borne by the forks is substantially reduced. This can be overcome with a combination of additional machine weight, extra counter weight and stabiliser or jack legs mounted in the front of the forklift. However, truck mounted forklifts must be of lightweight construction in order to ensure that they can be mounted on the carrying vehicle. It is therefore advantageous to employ means to increase forklift capacity without increasing the forklift weight.
A pantograph reach system and telescopic forks tilt from the mast or fork carriage. This results in a magnification of tilt moment as the reach of the forks is extended from the upright mast. The practical effect of this is increased tilt stresses and reduced control of the tilt function.
Further problems associated with both pantograph reach systems and telescopic forks are increased costs. Telescopic forks whilst being the most compact of the above three systems are an extremely expensive component for forklift trucks. The means by which the pantograph system operates requires a duplication of components, for example linkage pieces, channels, bearings and so forth to operate. Not only does this increase the cost of the forklift truck, it also creates additional weight that the forklift must counterbalance in order to operate effectively at extended reach. Furthermore the pantograph system forms a substantially increased overhang when the forklift is mounted on a carrying vehicle. This causes a problem due to strict road transport regulations for carrying vehicles such as trucks or lorries.
Each of the aforementioned problems is of increased importance when the forklift is required to reach across a trailer bed to offload a pallet without moving the forklift to the other side of the trailer. This is known as a double reach system. These systems normally comprise one or more of the aforementioned systems for examples, a combination of telescopic forks attached to a moving mast system, telescopic forks attached to a pantograph system or a pantograph system used in conjunction with a moving mast system.
Although this linkage system is mainly described in relation to truck mounted forklifts, conventional reach systems are also used for various warehouse forklifts and straddle trucks. In this application, regular pantograph reach systems are commonly used but do cause restriction when entering racking systems. This is especially evident on a double deep pantograph reach truck where loads must be accessed two deep in warehouse racking systems. These racking systems are generally built to maximise capacity and therefore use the minimum allowable spacing between racking shelves. This causes problems for conventional pantograph reach systems as another set of channels is mounted on the fork carriage and would therefore need much increased space between the shelves when accessing the inner pallet. For this reason manufacturers use a double pantograph system to keep the required height clearance down, however this comes with much increased cost, complexity, and load overhang. In addition, these systems are less rigid, have more moving parts and very much restrict visibility. However this application requires the fork carriage tilt angle to remain constant throughout the transition between fully retracted to fully extended which was a problem for previous low profile linkage system designs.